Summary
I’ve exported data from a test Slack server (having around 4 users, only a few channels and messages) and imported the data into Mattermost. Since I have the Business+ plan, the export included DMs and private channels too. The Mattermost server was a fresh install.
The issue is that users are unable to send messages to direct channels that were part of the Slack export. The message history loads, but the input itself is disabled and the following message is shown: Something went wrong while loading the component. Please wait a moment, or try reloading the app. Reloading the app, however, does not fix the issue, unfortunately. What’s more interesting is that users can send message to direct channels that were not part of the Slack export. So to sum it up:
User A Exists in Slack export
User B Exists in Slack export
User C Does not exist in Slack export
User A and B exchanged DMs, which is also included in the Slack export
User A and B can send message to User C. User A and B cannot, however, send direct messages to each other, as described above.
The output of mattermost version:
Version: 11.7.2
Build Number: 26431767452
Build Date: Tue May 26 04:16:29 UTC 2026
Build Hash: 70c68d0cf6357e2b5c6b66bcc6ffc03bd13d1910
Build Enterprise Ready: true
Steps to reproduce
1) The Mattermost server was installed via this Proxmox community script.
2) I followed the official migration guide.
Mattermost: v11.7.2
mmctl: v11.7.2
mmetl: v0.4.0
Expected behavior
Users should be able to send DMs to other users.
Observed behavior
I check the devtools console and network tabs looking for errors, but surprisingly, couldn’t see anything interesting. I did export data from the Mattermost server after playing around with it, and noticed that the participants of direct channels, that were not part of the Slack export had a property, called scheme_user (set to true). This property was not present on participants transformed by mmetl from the Slack export. Adding this property to all channel participants and importing it into a fresh Mattermost install fixed the issue, all users were able to send messages to the appropriate direct channels.
Example direct channel without scheme_user (as generated by mmetl from the Slack export):
{"type":"direct_channel","direct_channel":{"members":["tamas.adam","peter.varadi"],"participants":[{"username":"tamas.adam","msg_count":24,"msg_count_root":24,"last_viewed_at":1779351726217},{"username":"peter.varadi","msg_count":24,"msg_count_root":24,"last_viewed_at":1779351726217}],"header":""}}
The same direct channel with scheme_user. Importing this results in a usable direct channel:
{"type":"direct_channel","direct_channel":{"members":["tamas.adam","peter.varadi"],"participants":[{"username":"tamas.adam","msg_count":24,"msg_count_root":24,"last_viewed_at":1779351726217,"scheme_user":true},{"username":"peter.varadi","msg_count":24,"msg_count_root":24,"last_viewed_at":1779351726217,"scheme_user":true}],"header":""}}
Any help with this would be appreciated, I’m not sure if this is genuinely a bug in Mattermost or mmetl or if I haven’t configured some fallback somewhere.